Sometimes, the fuel filters do appear to be empty but fuel is actually flowing through them. Pull the pipe off the carburettor inlet and hold it over a suitable receptacle and with the tap (if fitted) turned on, and the tank cap in place watch the flow for at least half a minute . If the flow slows or stops repeat with the tank cap loosened or removed. If that improves things, check the breather hole in the cap. Blowing back into the tank outlet may improve things but the crud is still in there waiting to do it again. If that's the cause take the tank off and flush it properly.